Owing to the large solid angle that is captured, scattered radiation plays a significant part in computed tomography with multi-line and flat panel detectors. Even if anti-scatter grids or special collimators are used to reduce the scattered radiation the intensity thereof can still be considerable and even exceed the primary intensity. Artifacts consequently result as well as quantitative falsifications following image reconstruction, with errors in excess of 100 HU (=Hounsfield units). Additional scattered radiation correction methods are therefore required.
Spectral radiation hardening effects also occur in principle in the case of CT with X-ray tubes which emit a polychromatic photon spectrum. These effects manifest themselves following image reconstruction as what are known as “cupping”, i.e. trough-like density reduction from the outside in, even with homogeneous material, such as water in a cylinder. Image errors of up to >100 HU which, for example, occur as bar-like artifacts between prominent bone structures result in this case as well. In terms of appearance the falsifications generated by hardening or scattered radiation are very similar and it is almost impossible to differentiate them.
It is known that the presence of bones in the beam path affects hardening and scattered radiation, albeit differently. With radiologically different materials, such as soft tissue and bone, the scattered radiation depends not only on the fractions of the two materials but also on their sequence in the beam path. This means that the scattered radiation intensity can for example be significantly different with the same primary attenuation and with a combination of water and bone if exactly opposing projections, for example anterior→posterior and posterior→anterior, are examined. Hardening on the other hand is independent of the sequence of materials.
Post-reconstructive iterative hardening correction algorithms that take account of bone have already been published a long time ago. In this case reference is made by way of example to the documents P. Joseph, R. Spital: “A Method for Correcting Bone Induced Artifacts in Computed Tomography Scanners”, J. Comp. Assist. Tomogr., January 1978, vol. 2, pp. 100 to 108 and P. Joseph, C. Ruth, “A Method for Simultaneous Correction of Spectrum Hardening Artifacts in CT Images Containing Both Bone and Iodine”, Med. Phys. vol. 24 (10), October 1997, pp. 1629 to 1634.
From the document M. Zellerhof, B. Scholz, E.-P. Ruhrnschopf, T. Brunner: “Low contrast 3D reconstruction from C-arm data”, Proceedings of SPIE. Medical Imaging 2005, vol. 5745, pp. 646 to 655 a hardening correction of CT projection data is also known as pre-reconstructive “water correction” in which in simplistic terms it is assumed that the attenuation of the X-radiation is caused solely by water-equivalent material. The pre-reconstructive scattered radiation correction method that is also outlined therein is based on similar simplified assumptions.